X-ray analysis of materials has been used as an analytical tool many times during the last few decades. Conventional x-ray diffraction has been performed both at high temperatures and at cryogenic temperatures, particularly for the study of phase transformations by observation of changes in lattice parameters. Such techniques involve diffraction from a limited area of the sample. Subgrains within this limited area can contribute to the half-width of the diffracted peak. analysis of rocking curves can give information about the angles of subgrains. However, an observer does not know the actual shape and distribution of the subgrains but only that they are present. X-ray topography, however, is a method of x-ray analysis that yields a picture of the sample surface with the subgrains actually delineated. In the Berg-Barrett topographic method, a divergent beam floods the crystal surface at a small incident angle.
A high-resolution photographic plate is placed as close as possible to the sample surface to record the image produced by the x-ray beam diffracting from the sample surface. The Lang method of x-ray topography involves using a highly collimated x-ray beam, thus precluding the possibility of illuminating the entire sample surface at one time. The Lang camera provides a means of translating the sample and photographic plate back and forth relative to the x-ray beam, thus scanning the entire surface. For the Lang technique, the spatial resolution of the recorded image does not depend critically on the sample-to-plate distance, since the beam is highly collimated, unlike the diverging beam of the Bert-Barrett technique.
Conventional x-ray diffraction experiments, when performed on samples maintained at cryogenic temperatures, require continuous pumping of an evacuated chamber to eliminate heat transfer by conduction. Such experiments often require a somewhat bulky sample chamber. The vacuum pump hose must, of course, be connected to the chamber to be evacuated. This type of arrangement is not suitable for x-ray topography for a number of reasons. For Lang x-ray topography, the sample must be capable of translational movement relative to the x-ray beam, and hence the entire sample chamber must be translated during the scanning process. The vacuum pump hose would produce a torque which would tend to rotate the sample relative to the x-ray beam, thus destroying the critical angular alignment of the sample with the x-ray beam. In addition, the vacuum pump vibrations would be transmitted to the sample chamber by the vacuum pump hose. These vibrations would be sufficient to degrade the resolution of the topographic image produced by either the Berg-Barrett method or the Lang technique used in the reflection mode. In addition, the samle chamber vibrations would make it very difficult to place the photographic plate within a millimeter of the sample, as requred for the Berg-Barrett technique of x-ray topography.